If you’re one of the many people who wanted both the convenience of having IS and the benefits of having f/2.8, here’s some good news: there is indeed a 24-70mm f/2.8 IS on the way.
Craig Blair of Canon Rumors has confirmed with two sources that there are working prototypes of that lens floating around in photographers’ hands, and that it’s a lens Canon is definitely planning to release.

The bad news? It might be some time before it’s officially announced. Blair gives an 18-month time frame, saying that late 2013 or early 2014 is the earliest we might see Canon officially take the wraps off this lens.

As we wrote in October, the 24-70mm lineup is starting to look more and more like Canon’s 70-200mm lineup, with a number of models based around different apertures and IS capabilities. However, Blair writes that there are no plans for a non-IS version of the 24-70mm f/4, so the 24-70mm lineup will have at least one fewer lens than the 70-200mm one:

Before this new generation of lenses, there was just one Canon lens that occupied the 24-70mm range: “The Brick.” No word yet on the specs or pricing of the upcoming lens, but we’ll guessing it’ll lighten your wallet a bit more than the current non-IS version.